Porphyria cutanea tarda (PCT), the most common form of porphyria in humans, is due to sub-normal activity of uroporphyrinogen decarboxylase (URO-D), a cytosolic enzyme in the heme biosynthetic pathway. Our preliminary data indicate that diminished activity of URO-D in the livers of rodents with experimentally induced porphyria and in humans with PCT is due to the generation of a low-molecular-weight inhibitor. Generation of the inhibitor can be initiated by both environmental and genetic factors, and involves an iron-catalyzed reaction. We hypothesize that the inhibitor is generated from oxidized hydroxymethyl bilane, an early intermediate in the heme biosynthetic pathway. We propose to chemically synthesize this inhibitor and to establish that an identical compound is generated in vivo in rats and mice with experimental porphyria. We have identified the presence of a URO-D inhibitor in liver biopsy samples from subjects with PCT. We plan to confirm this finding in additional subjects and to establish that the inhibitor occurs only in patients with PCT by attempting to detect inhibitor in liver biopsy samples from patients with hepatitis C, alcoholic liver disease, and hemochromatosis. Murine models of PCT will be developed in carefully characterized genetic backgrounds by intercrossing a strain we have generated by disrupting one allele of the URO-D gene (URO-D+/- mice). These animals have been intercrossed with homozygous hemochromatosis knockout animals (HFE-/-) and the resultant animals develop PCT without the need for any exogenous agents. We plan introduce null alleles of cytochrome P4501A2 (CYP1A2) and null alleles of peroxisome proliferator activated receptor (PPARa) onto the URO-D+/-:HFE-/- background to define the genetic components required for the development of experimental PCT. Our findings in PCT could serve as a paradigm for other dominantly transmitted porphyrias where clinical expression cannot be simply explained by half-normal activity of non-rate-limiting heme biosynthetic enzymes.